Field of the Invention
The present invention relates to a multilayer, sealable plastic film.
Description of the Related Art
Japanese Patent Publication 41 892/79 describes a heat-seal film which is composed of an ethylene/acrylate copolymer and a styrene/butadiene block copolymer; Japanese Patent Publication 137 242/78 uses for this purpose a blend of styrene/butadiene block copolymer and/or high impact polystyrene and an ethylene/vinyl acetate copolymer. Similar proposals for a sealable film which permit polystyrene containers to be sealed are made in the further publications
JA-047 462/78 PA1 JA-011 761/87 PA1 JA-133 735 87 PA1 JA-147 650/89 PA1 DE-A-2 944 344. PA1 A: a top layer A of a polymer which is dimensionally stable at elevated temperatures and has a melting point above 200.degree. C., PA1 B: an adhesion promoter B of from 30 to 95% by weight of a styrene/butadine block copolymer, of a styrene/butadiene block copolymer which contains up to 95% by weight of a styrene component and/or high impact polystyrene and from 5 to 70% by weight of a polymer which is incompatible with polystyrene and contains polar groups, the percentages being based on B, the styrene/butadiene block copolymer containing up to 95% by weight of polystyrene or high impact polystyrene, PA1 C: a substrate layer C of from 5 to 100% by weight of a styrene/butadiene block copolymer and up to 95% by weight of polystyrene and/or high impact polystyrene, PA1 D: if required, a barrier layer D which may correspond to the top layer A may be incorporated between layer C and layer E and PA1 E: a sealing layer E whose composition essentially corresponds to that of the adhesion promoter B. PA1 Simple production on existing coextrusion units PA1 Good recycling properties of the punch waste and the finished cup PA1 Good sealing and peelability PA1 No delamination during peeling, no residues on the cup lip PA1 High toughness (no tearing during the peeling process, in particular in the case of hooded lids; no indentation during transport) PA1 No migration of ingredients (for example solvents) or undesirable permeation (for example O.sub.2, H.sub.2 O ) PA1 Existing sealing units can be easily converted from aluminum to plastic PA1 Little sensitivity of the peelability and sealing to the sealing conditions, in particular temperature fluctuations PA1 No adhesion to the sealing jaws PA1 Can easily be printed PA1 No change in the cup material PA1 No blocking of the films and of the lid PA1 Sterilizable (for example with H.sub.2 O.sub.2)
The adhesion and toughness of the films described are, however, insufficient.
EP-A-42 831 describes a lid system which comprises a sealable (=substrate) layer consisting of poly(ethylene/vinyl acetate) and homopolystyrene and a polyester top layer. These layers are bonded by means of an adhesive which is not described in detail. Polystyrene cups are sealed therewith in a peelable manner.
To permit easier recycling, plastic lids are to replace aluminum lids. However, the known systems are not suitable for this purpose since they are for the major part (&gt;80%) composed of materials which are incompatible with polystyrene. Moreover, the adhesives used are frequently crosslinked or, where they are thermoplastic, consist of polyurethane or polyacrylate, which together with the polyester substrate layer used further restricts the possibility of recycling. This means that recycling together with polystyrene (the usual cup material) is not possible. Another possibility would be to add recycled material to the sealable layer. Experience has that it is precisely the peelability which is very sensitive to the composition of the components. The addition of a recycled material contaminated with polyester and adhesive would no longer ensure closure of polystyrene containers to meet quality assurance standards.
DE-A 39 21 677 describes a complete lid system comprising a heat-seal layer, a substrate layer and a top layer, which may furthermore be bonded by means of adhesion promoters. The polymers on which the sealing and top layer are based are high impact polystyrene and styrene/butadiene block copolymers. Experience has shown that the combination of toughness, adhesion and peelability is not yet sufficient to permit aluminum lids to be replaced. In addition, the coextruded top layer frequently requires an adhesion promoter, owing to a strong tendency toward delamination. This requires the use of expensive coextrusion units.
Furthermore, the nature of the sealing layer used results in small residues remaining on the cup lip, which may cause the end user to have doubts about the cleanness of the packed food.